This invention relates to a secondary battery comprising a nonaqueous electrolyte, containing an improved positive active material, and a method of manufacturing the same.
Secondary batteries containing nonaqueous electrolytes with negative electrodes comprising lithium, lithium alloy or lithium compounds are expected to be of high voltage and high energy density, and have been the subject of intensive research.
Compounds such as V.sub.2 O.sub.5, Cr.sub.2 O.sub.5, MnO.sub.2, TiS.sub.2, etc. have been used as a positive active material in a secondary battery comprising a nonaqueous electrolyte.
Recently, Thackeray et al. reported that Li.sub.x Mn.sub.2 O.sub.4 could serve as a positive active material in a secondary battery comprising a nonaqueous electrolyte (Material Research Bulletin, 18 (1983), 461-472).
The relationship of the x value of Li.sub.x Mn.sub.2 O.sub.4 and the electromotive force is shown in FIG. 2. These data demonstrate that the potential curve has two flat parts, in the vicinity of 4 V and of 2.8 V. Accordingly, Li.sub.x Mn.sub.2 O.sub.4 can be used in a lithium secondary battery of the 4 V class, if it is charged and discharged along the flat part of 4 V, within the range of 3 V to 4.5 V. The opencircuit potential against x-value of another positive active material, Li.sub.x CoO.sub.2, is given in FIG. 3. Since the potential curve exhibits flat parts in the vicinity of 4.0 V and of 1.2 V, Li.sub.x CoO.sub.2 can also be used in a lithium secondary battery of the 4 V class, by charging and discharging at the flat part of 4 V. Regarding the negative electrode, lithium metal has been the target of research investigations. However, a negative electrode of lithium has disadvantages in that when charged, a dendrite lithium is generated on the surface thereof and, following repeated cycles of charging and discharging, a low charging-discharging efficiency or shortcircuiting within the battery was observed which was due to the contraction of the positive electrode. This led to research on negative active material holders such as carbon, aluminum or its metal or alloy, or other kinds of oxides which absorb and desorb lithium in the absence of accumulation of dendrite.
In a secondary battery comprising carbon or aluminum as the negative active material holder, LiMn.sub.2 O.sub.4, LiMnO.sub.2, LiCoO.sub.2, LiNiO.sub.2 LiFeO.sub.2 or .gamma.-LiV.sub.2 O.sub.5 as the positive active material, and a non-aqueous organic electrolyte such as lithium perchlorate organic solvent solution, the first charging causes lithium ions to be released from the positive active material and to be absorbed by the negative active material, and the subsequent discharge causes the transfer of lithium from the negative active material to the positive active material. The lithium ions taking part in the battery reaction are confined solely to the ions which existed in the positive electrode at the beginning of the cycle. These ions therefore determine the capacity of the battery. In a battery comprising the above described negative active material, the lithium which is absorbed by the negative active material during the first charging cycle is not necessarily released during the subsequent discharging event. (The amount of lithium absorbed by the negative active material is greater than that released by the negative active material: this is the charging-discharging capacity difference). When lithium is taken up by the negative active material but does not participate in the remaining events in the battery, the number of lithium ions capable of cycling during charging and discharging is decreased, resulting in a decline in battery capacity.
In order to enhance the lithium capacity of the negative active material holder, use of Li.sub.x Mn.sub.2 O.sub.4 , Li.sub.x MnO.sub.2 Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2, Li.sub.x FeO.sub.2, or .gamma.-Li.sub.2 V.sub.2 O.sub.5 (x&gt;1) as the positive active material containing an excess of lithium ions has been proposed.
In order to synthesize these materials, methods have been developed to add excess lithium salt to the raw material of each compound, or alternatively the lithium content of the positive active material can be increased by the addition of butyllithium (Japanese Patent Application Laid-Open, No. Toku-Kai-Hei 2-265167). Regarding the former method, however, Li.sub.x Mn.sub.2 O.sub.4 , Li.sub.x MnO.sub.2, Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2, Li.sub.x FeO.sub.2 or .gamma.-Li.sub.x V.sub.2 O.sub.5 (x&gt;1) obtained by the addition of excess lithium by heating, react with moisture in the air and decompose to form LiMn.sub.2 O.sub.4, LiMnO.sub.2, LiCoO.sub.2, LiNiO.sub.2, LiFeO.sub.2 or .gamma.-LiV.sub.2 O.sub.5 and LiOH or the like. Thus, these compounds (Li.sub.x Mn.sub.2 O.sub.4 etc.) cannot tolerate the application of water as a solvent during pulverization or classification after synthesis. Instead, the processes must be carried out in air, or inert gas without moisture. Regarding the second method wherein the positive active material is doped with lithium by dipping the material powder, after synthesis, into butyllithium as a lithium-adding liquid, the positive active material containing excessive lithium is unstable in a moist atmosphere. Thus, these steps must also be carried out in a dry atmosphere using a nonaqueous solvent.